The 24-year-old suffered a cardiac arrest in Monday's reserve fixture with KRC Genk and, after being resuscitated on the pitch, was in a medically-induced coma for days until his death was announced.
Former Bolton Wanderers player Fabrice Muamba was 23 when he went through the exact same situation in March 2012 against Tottenham, except he survived the cardiac arrest, and he passed on his respects.
Muamba's former club Arsenal passed a message of support to its 5.71 million followers following the central defender's death.
Horrible news that Gregory Mertens has passed away, only 24 years old. #RIP— fabrice muamba (@fmuamba) April 30, 2015
Dreadful news that Gregory Mertens has passed. ❤️#r.i.p— Micah Richards (@MicahRichards) April 30, 2015
Premier League leaders Chelsea, the home of Belgian goalkeeper Thibaut Courtois and playmaker Eden Hazard, also paid tribute to Mertens.
Manchester City are familiar with heart-related football tragedy due to Marc-Vivien Foe's death in 2003 while playing for Cameroon, which resulted in his final club paying their respects by retiring the No.23 shirt.
All at Chelsea are saddened by the death of Gregory Mertens. Our thoughts are with his family friends and @KSCLokeren. May he rest in peace.— Chelsea FC (@ChelseaFC) April 30, 2015
Although Mertens never featured Belgium's senior team, the centre-back played for the country's youth teams up until Under-21 level and the official account of his home nation's FA also paid their respects.
Our thoughts are with the friends and family of @KSCLokeren's Gregory Mertens at this sad time.— Manchester City FC (@MCFC) April 30, 2015
Thousands of Belgians poured out support for Mertens and his family after the news broke he had died, and national team stars Romelu Lukaku, Thibaut Courtois and Kevin De Bruyne were among those.
Gregory, je glimlach vergeten we nooit. Nous n’oublierons jamais ton sourire. We will never forget your smile. RIP. pic.twitter.com/EihAVUc5ij— Belgian Football (@Belgianfootball) April 30, 2015
Rest In Peace Gregory Mertens ... Gone to soon— R.Lukaku Bolingoli10 (@RomeluLukaku9) April 30, 2015
RIP Gregory Mertens! Veel sterkte aan familie en vrienden! #gonetosoon— Thibaut Courtois (@thibautcourtois) April 30, 2015
Rest in peace my friend Greg! Gone to soon— Kevin De Bruyne (@DeBruyneKev) April 30, 2015